I guess it's for collectors who want a physical copy of the game. It seems rather cheap not to include enough discs to fit the whole game though. Would it really be 30 GB in size even when compressed?

I'll never understand this mentality. If you don't like the game, you can choose not to buy it, but don't try to prevent other people from playing a game they enjoy. You would lose absolutely nothing if the Bethesda Fallout games were available on GOG.

but I can't stopping thinking that is totally anti-environment.

I mean, a full copy of the game has a sense and purpose, but a worthless DVD is just more trash for the environment (and lets not speak about costs for the industry because they will still charge the same amount even with the saving).